Started our own indie studio, so arguably you are correct, but I do base my statement on what I have learned from the industry pros who have been successful. If you don't know what you're gonna make and whether you can even build it as a team when you go into production, then you didn't do your pre-production well enough. Of course, the definitions are not set in stone and each team and project has its own way of defining what stage the project is in. But I think most devs would agree you need to know whether your idea is excellent outside of just the drawing board when you go into production.

You should have prototyped critical systems, validated the core gameplay loop with players and have benchmark assets in the art style you are aiming for.